Fiber optic sensors employ the fact that environmental effects, such as pressure, strain, vibration, and temperature, can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. Advantages of fiber optic sensors include their light weight, small size, passive nature, energy efficiency, and ruggedness. In addition, fiber optic sensors have the potential for very high sensitivity, and wide bandwidth. Yet further, certain classes of sensors can be distributed along the length of an optical fiber so that an appropriate interrogation system can be employed to monitor selected environmental parameters at multiple locations at the same time.
One application in which fiber optic sensors can be used is subterranean seismic surveying. In general, subterranean surveying can be used to determine the content of a subterranean structure (e.g., a hydrocarbon-bearing formation), which can be underneath a land surface or under a water bottom surface (e.g., seabed). Marine subterranean surveying involves deploying sensors that are towed through a body of water, or deployed on the water bottom surface. Land subterranean surveying involves deploying the sensors on the land surface or in a wellbore.
Seismic subterranean surveying employs seismic sources to generate seismic signals that are propagated into a subterranean structure. The propagated seismic signals are reflected from subterranean elements in the subterranean structure, where the reflected signals are detected by the seismic sensors. The data collected by the seismic sensors are then processed to determine characteristics of the subterranean structure. One type of survey that can be performed is referred to as vertical seismic profiling, where a seismic source located at the surface generates seismic signals that are detected by sensors that are located in a wellbore. Alternatively, the seismic source can be located in the wellbore and the sensor can be located at the surface or in another well. In the latter case, the technique is referred to as cross-well seismic surveying.
Fiber optic sensors also can be used in systems that are arranged to monitor certain parameters (e.g., pressure, temperature, strain, etc.) in a region of interest. For instance, when deployed in a hydrocarbon well, a fiber optic sensor can provide indications of characteristics of production fluids, such as fluid composition, density, viscosity, flow rate, etc. Various drilling, production and remedial operations can then be performed based on the information derived from the monitored parameters.